Introduction Reticulated platelets (RP) are the youngest forms of circulating platelets, which contain residual RNA, and therefore constitute the platelet equivalent of red cell reticulocytes. The first description was made in 1969, by direct visualization in peripheral blood film in an animal model of blood loss. The first study investigating the clinical application do not appear until 1990, when it is possible to use flow cytometry for their identification. RP measurement by flow cytometry is not a standardized technique. Several dyes for mRNA stain have been used. Two substrates for RP determination, whole blood or platelet rich plasma have been published and different incubation times have been described Methodology From 2002 to 2005, we prospectively enrolled both admitted and out patients being attend at our Hospital with thrombocytopenia, fewer than 100x109 /L, confirmed after peripheral blood film review and a control group of non thrombocytopenic patients. A direct, whole-blood, dual–labelling flow cytometric method was used. Direct, whole-blood double coverage was achieved using a monoclonal anti-glycoprotein III antibody (CD 61 PerCP) for platelet identification and thiazole orange (Retic-count) as platelet mRNA stain. The automatic determination of PR, was measured by determining the immature platelet fraction (IPF) by autoanalyzer Sysmex XE 2100. Objectives and Results Objective 1 To assess whether the determination of reticulated platelets by flow cytometry using double labeling is a technique capable of differentiating different causes thrombocytopenia. The results of this first study were published in European Journal of Internal Medicine (M. Monteagudo Jiménez, MJ Amengual Guedán, L Muñoz Martín, JA Soler Campos, I Roig Martínez, C Tolosa Vilella. Measurement of reticulated platelets by simple flow cytometry: an indirect thrombocytopoienic marker. Eur J Intern Med 2006 Dec; 17 (8): 541-4). Objective 2. Assess whether the determination of reticulated platelets by flow cytometry from whole blood can be a useful initial screening test for the etiological diagnosis of thrombocytopenia. The results of this second study were published in Quarterly Journal of Medicine (Monteagudo M, Amengual MJ, Muñoz L, Soler JA, Roig R, Tolosa C. Reticulated platelets as a screening test to Identify thrombocytopenia aetiology QJM. 2008 Jul; 101 (7) :549-55). Objective 3. To analyze the correlation between the determination of reticulated platelets by flow cytometry and the number of megakaryocytes in bone marrow determined by bone marrow aspiration. The results of this third study were published in Medicina Clínica (Monteagudo M, Lucchetti G, Amengual MJ, Muñoz L, Soler JA, Roig R, Tolosa C. Relationship between reticulated platelets and megacaryocyte number in thrombocytopenic patients. Med Clin (Barc). 2009 June 20, 133 (3) :81-5). Objective 4. To assess the correlation between PR values determined by flow cytometry with the results of the immature platelet fraction (IPF), automatically determined by the autoanalyzer Sysmex 2100. The results of this latest study were published in European Journal of Haematology (Pons I, Monteagudo M, Lucchetti G, Muñoz L, Perea G, Colomina I, Guiu J, Obiols J. Correlation between immature platelet fraction and reticulated platelets. Usefulness etiology in the diagnosis of thrombocytopenia. Eur J Haematol 2010 August, 85 (2) :158-163). Conclusions 1. The determination of reticulated platelets from whole blood using double labeling, flow cytometry and without sample manipulation is a technique easy to perform and reproducible. 2. The determination of reticulated platelets by this technique allow to differentiate between thrombocytopenia of central origin of thrombocytopenia of peripheral origin. 3. Reticulated platelet values above 11% with this methodology, have a high sensitivity (93%) and good specificity (85%) for the diagnosis of peripheral thrombocytopenia with high megakaryocytic marrow activity. 4. Should be considered the possibility of false negatives, but especially false positives, in some patients with myelodysplastic syndromes and some acute leukemias of lymphoid or myeloid. 5. Reticulated platelets in peripheral blood are a surrogate marker for megakaryocytic marrow activity. 6. The presence of peripheral reticulated platelets above 11% in isolated and acute thrombocytopenia, is well correlated with an elevated number of megakaryocytes in bone marrow 7. There is good correlation between the determination of reticulated platelets by flow cytometry carried out according to our methodology, and values of immature platelet fraction (IPF) performed automatically by the autoanalyzer Sysmex IPF-2100. 8. . IPF values obtained in our patients tend to be higher than the values of reticulated platelets measured by flow cytometry 9. The simplicity of implementation of the IPF, reproducibility and good correlation with flow cytometry, makes its determination on an useful screening test for thrombocytopenic patient, provided that we take into account the overall assessment of the patient.